Binder removal in selective laser sintering

ABSTRACT

A method of fabricating an article, such as a prototype part or a tooling for injection molding, by way of selective laser sintering, using a composite powder system of a metal and/or ceramic powder with a polymer binder comprising thermoplastics and thermoset polymers, and a metal hydride powder to form a “green” article. After removal of unfused material from the green article it is placed in an oven or furnace in a non-reactive atmosphere such as, for example, nitrogen or argon, for subsequent heat treatment to decompose and drive off the binder and sinter the metal substrate particles prior to infiltration by a metal with a lower melting point. During the critical step of decomposing the binders, the metal hydride begins to decompose also and releases an in-situ concentration of hydrogen gas that creates the reducing conditions necessary to thoroughly decompose the polymer fragments so that the hydrocarbon fragments can escape the skeleton structure of the article. It has been found that even with higher loadings of binders, leading to higher desired green strengths, the decomposition of the metal hydride eliminates the blistering phenomena associated with high loadings of some binders.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The field of solid freeform fabrication (SFF) of parts has, inrecent years, made large improvements in providing high strength, highdensity parts for use in the design and pilot production of many usefularticles. “SFF” generally refers to the manufacture of articles in alayer-wise fashion directly from computer-aided-design (CAD) databasesin an automated fashion, as opposed to conventional machining ofprototype articles from engineering drawings. As a result, the timerequired to produce prototype parts from engineering designs has reducedfrom several weeks, using conventional machinery, to a matter of hours.

[0003] 2. Description of the Relevant Art

[0004] One example of an SFF technology is the selective laser sinteringprocess practiced by systems available from 3D Systems, Inc. ofValencia, Calif. According to this technology, articles are produced inlayer-wise fashion from a laser-fusible powder that is dispensed onelayer at a time. The powder is fused, or sintered, by the application oflaser energy that is directed to those portions of the powdercorresponding to a cross-section of the article. After the fusing ofpowder in each layer, an additional layer of powder is then dispensed,and the process repeated, with fused portions of later layers fusing tofused portions of previous layers (as appropriate for the article),until the article is complete. Detailed description of the selectivelaser sintering technology may be found in U.S. Pat. No. 4,863,538 andU.S. Pat. No. 5,017,753, both assigned to Board of Regents, TheUniversity of Texas System, and in U.S. Pat. No. 4,247,508, expired, toHousholder; all incorporated herein by this reference in pertinent part.The selective laser sintering technology has enabled the directmanufacture of three-dimensional articles of high resolution anddimensional accuracy from a variety of materials including nylons,polystyrenes, and composite materials such as polymer coated metals andceramics. Examples of composite powder materials are described in U.S.Pat. No. 4,944,817, U.S. Pat. No. 5,076,869, and in U.S. Pat. No.5,296,062, all assigned to Board of Regents, The University of TexasSystem, and incorporated herein by this reference.

[0005] A related SFF technology, referred to as 3-Dimensional (3D)Printing, is described in U.S. Pat. Nos. 5,340,656 and 5,387,380. From acomputer (CAD) model of the desired part, a slicing algorithm drawsdetailed information for every layer. Each layer begins with a thindistribution of powder spread over the surface of a powder bed. Using atechnology similar to ink-jet printing, a binder material selectivelyjoins particles where the object is to be formed. A piston that supportsthe powder bed and the part-in-progress lowers so that the next powderlayer can be spread and selectively joined. This layer-by-layer processrepeats until the part is completed. Following a heat treatment, unboundpowder is removed, leaving the fabricated part.

[0006] As this technology has evolved, SFF has increasingly been usednot only to make prototype parts but also to make final useful parts aswell as tools or molds that can be used to make multiple parts. Adeveloping trend is to fabricate such parts, tools, or molds with an“indirect” process that uses a powder of metal and/or ceramic particleseither coated by or blended with a polymer, from which a “green” articleis fabricated by selective laser sintering to bind the particles to oneanother. The green article is then heated to a temperature above thedecomposition temperature of the polymer, which both drives off thepolymer and also binds the metal and/or ceramic substrate particles toone another to form an intermediate porous article. The porous articlecan then be infiltrated with another material such as a lower meltingtemperature metal to give a fully dense article with desirableproperties. The green article can also be fabricated with 3D printing.

[0007] Some examples of the use of these approaches for functionalapplications are described, for example, in U.S. Pat. Nos. 5,433,280,5,544,550, and 5,839,329 to Smith et al. These describe the use ofselective laser sintering a tungsten carbide-polymer composite powder togenerate a “green” drill bit which is then infiltrated in a furnacecycle with a copper alloy to generate a fully functional drill bit fordown hole oil exploration. Another commercial application of theseindirect approaches is a product called ProMetal by ExtrudeHone.Utilizing the 3D Printing technology described above, ProMetal buildsmetal components by selectively binding metal powder layer by layer. Thefinished structural skeleton is then sintered and infiltrated withbronze to produce a finished part that is 60% steel and 40% bronze andis used for injection molding tools or final metal parts. Anothercommercial example is 3D Systems' ST-100 system, which uses selectivelaser sintering of a steel polymer composite powder to generate a greenarticle which is subsequentially put through a furnace cycle thatremoves the polymer binder and infiltrates the metal skeleton withbronze to create a functional fully dense article that can also be usedfor injection mold tools or final parts.

[0008] As is well known in the art, the structural strength of the greenarticle is an important factor in its utility, as weak green articlescannot be safely handled during subsequent operations. Another importantfactor in the quality of a prototype article is its dimensional accuracyrelative to the design dimensions. However, these factors of partstrength and dimensional accuracy are generally opposed to one another,considering that the densification of the powder that occurs in thesintering of the post-process anneal also causes shrinkage of thearticle. The polymer content of a metal and/or ceramic composite powderdescribed above could be increased in order to provide higher green partstrength, but the shrinkage of the part in post-process anneal wouldincrease accordingly. As a result, compromises between article strengthand dimensional stability must be made in the design of the compositepowder system.

[0009] Some drawbacks of conventional composite powders incorporatingthermoplastic polymer binders have been observed. In the post-processanneal of green articles using such binders, creep deformation has beenobserved as the article is heated to a temperature above the glasstransition temperature of the polymer binder, but below thedecomposition temperature at which the binder is released. The viscosityof the polymer decreases to such an extent that the metal or ceramicsubstrate particles slide past one another under the force of gravity.Not only do the dimensions of the article change as a result of thiscreep deformation, but also this dimensional change is not uniform inthat taller features deform by a larger extent than do shorter features.This non-uniformity in deformation precludes the use of a constantshrinkage correction factor in the selective laser sintering fabricationof the green part, further exacerbating the difficulty of achievingdimensionally accurate articles of high density and strength.

[0010] Creep deformation has been observed to deform not only the heightbut also the shape of vertical features such as sidewalls. For example,vertical walls of mold cavities formed by selective laser sintering ofpolymer-coated metal powders, and having a thickness of 0.75 inches anda height of 1.5 inches, have been observed to bow outwardly as a resultof creep deformation. The dimensional accuracy of the infiltrated finalpart is, of course, severely compromised by such deformation.

[0011] To address this tendency of creep deformation, another prior arttechnique was developed that combined the use of a thermoplastic binderwith a thermoset binder. This is described in U.S. Pat. No. 5,749,041.In this approach a “green” part is formed by the selective lasersintering of a metal-polymer composite powder, in which the polymerbinder is a thermoplastic polymer. Following its fabrication, the greenarticle is infiltrated with a thermosetting material prior to heatingthe part. The thermosetting material may be an aqueous emulsion of across-linkable polymer with a cross-linking agent, or may instead be anaqueous emulsion of only the cross-linking agent. In the first case, thecross-linking agent reacts with the cross-linkable polymer in theinfiltrant to form a rigid skeleton for the green article; in the secondcase, the cross-linking agent reacts with the polymer binder of thegreen article to form the rigid skeleton. Following the formation of therigid skeleton, the article may be heated to decompose the polymer andsinter the metal substrate particles, followed by infiltration with ametal for added strength. This prior art approach enabled a solution tothe creep deformation problem but added significant time to the postprocessing of the part to dry out the article after the aqueousinfiltration step.

[0012] Another approach used commercially to avoid the aforementioneddrying step was to incorporate both a thermoplastic and thermoset binderin the formulation of the metal-polymer composite article. In onesuccessful version a phenolic type thermoset was combined with a waxbinder to give a system that gave adequate initial green strength and amore rigid skeleton for the green article. The green strength of thissystem though, while improved, still has resulted in unacceptablefailure rates due to breakage of green parts in handling. Thus thesearch for stronger green part systems has continued. The trend has beento use more polymer binder materials over time.

[0013] As the amount and complexity of binders has increased in thesemetal and/or ceramic polymer composite approaches, there has beenincreased difficulty in removing all of the polymer system bindersduring the decomposition and burn-out phase. The decomposition of thepolymer into smaller fragments should be complete enough to ensure thatthe bulk of the hydrocarbon fragments can escape the article skeletonbefore the infiltrating metal (copper or bronze, for example) enters theskeleton. If all of the hydrocarbon fragments do not escape, trappedones can lead to a phenomena of blistering on the surface of the finalarticle. In some systems the presence of too much residual carbon canalso impede the infiltration process. The presence of a reducingatmosphere, such as hydrogen or forming gas helps the polymerdegradation greatly but is a more expensive alternate than an inertnitrogen atmosphere.

[0014] Accordingly, there is a need for improving the efficiency of thedecomposition and removal of the polymer binder systems during the ovenor furnace cycle.

BRIEF SUMMARY OF THE INVENTION

[0015] It is therefore an aspect of the present invention to provide amethod of fabricating high density and high strength articles andtooling via SFF techniques from a metal and/or ceramic and polymercomposite powder with improved initial green strengths.

[0016] It is a further aspect of the present invention to provide such amethod using a composite powder that improves dimensional accuracy.

[0017] It is a further aspect of the invention to provide such a methodwhile avoiding blistering phenomena even in nitrogen atmospheres.

[0018] The invention may be incorporated into a method of fabricating anarticle, such as a prototype part or a tooling for injection molding, byway of selective laser sintering. According to the present invention,the selective laser sintering of a metal-polymer composite powder, inwhich the polymer binder may be a thermoplastic polymer or a combinationof thermoplastics and thermoset binders, forms a “green” part. Inaddition, a metal hydride powder is added to the metal-polymer compositeformulation. After removal of unfused material from the green part it isplaced in an oven or furnace in a non-reactive atmosphere such as, forexample, nitrogen or argon for subsequent heat treatment to decomposeand drive off the binder and sinter the metal substrate particles priorto infiltration by a metal with a lower melting point. During thecritical step of decomposing the binders, the metal hydride begins todecompose also and releases an in-situ concentration of hydrogen gasthat creates the reducing conditions necessary to thoroughly decomposethe polymer fragments so that the hydrocarbon fragments can escape theskeleton structure of the article. It has been found that even withhigher loadings of binders, leading to higher desired green strengths,the decomposition of the metal hydride eliminates the blisteringphenomena associated with high loadings of some binders.

[0019] The invention also includes a preform green article formed byselective laser sintering comprising a composite powder, the compositepowder being fused and comprising metal and/or ceramic particles,polymer particles and metal hydride particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other goals and advantages of the present invention will beapparent to those of ordinary skill in the art having reference to thefollowing specification together with the drawings, wherein:

[0021]FIG. 1 is a flow diagram illustrating a method of fabricating anarticle according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] According to the preferred embodiments of the present invention,three-dimensional articles of complex shapes may be made with highdimensional accuracy and good part strength both in its green state andalso as a finished article. It is to be understood that, while thepresent invention is particularly useful in the fabrication of prototypeinjection molds and tooling, the present invention may also be used toadvantage in the fabrication of prototype parts, such as used in themodeling of mechanical systems. Indeed, it is contemplated that theselective laser sintering process and the method of the presentinvention may be used to manufacture end use articles and partstherefor, particularly in custom or very limited runs, as economicspermit. As such, the use of the term “article” hereinbelow will be usedto refer either to a part (prototype or end-use), or to tooling forinjection molding, thus encompassing various eventual uses of thearticle.

[0023] Referring now to FIG. 1, a method of fabricating an articleaccording to a first embodiment of the invention will now be describedin detail. According to this embodiment of the invention, the methodbegins with process 10, which is the selective laser sintering of acomposite powder to form a “green” article. The term “green” refers tothe intermediate state of the article, prior to its densification aswill be described hereinbelow. The composite powder used in process 10according to this embodiment of the invention is a metal and/or ceramicpowder blended with or coated by a polymer binder system and alsoincludes a metal hydride powder. The polymer binder system may usethermoplastics, thermosets, or a combination thereof.

[0024] Selective laser sintering process 10 is preferably performed in amodern selective laser sintering apparatus, such as the VANGUARD systemavailable from 3D Systems, Inc. As described in the above-referencedpatents, process 10 fabricates the green article in a layer wisefashion, by dispensing a thin layer of the powder over a target surface,preferably in a controlled environment, and then applying laser energyto selected locations of the powder layer to fuse, or sinter, the powderthereat. According to the present invention, wherein the powder is acomposite powder of metal or ceramic particles, polymer binder particlesand metal hydride particles, and the powder particles are fused to oneanother by the melting and cooling of the polymer binder, rather than bysintering of the metal substrate particles (which would require veryhigh laser power). The selected locations of the powder layer correspondto those portions of the layer in which the article is to be formed, asdefined by a computer-aided-design (CAD) data base representation of thearticle. After the selective fusing of a layer, a subsequent layer isdisposed over the previously processed layer, and the selective fusingis repeated in the new layer at locations of the layer corresponding tothe CAD “slice” of the article to be formed therein. Those portions of alayer that overlie fused portions of the powder in the prior layer arebonded to the fused portions in the prior layer, such that a solidarticle results. The unfused powder in each layer serves as a supportmedium for subsequent layers, enabling the formation of overhangingelements in the article. As a result of process 10, the green article isformed to the desired size and shape.

[0025] It is contemplated that the particular settings and operatingparameters of the selective laser sintering system used in process 10may be readily selected by one of ordinary skill in the art. Theseparameters include such items as the laser power, laser scan rate,ambient chamber temperature, layer thickness and the like. Typically,the values of these operating parameters are optimized for a givencommercially-available powder, such as the composite powder describedabove, according to documentation provided by the system manufacturer.

[0026] Other thermal-based additive processes may alternatively be usedto form the green article. For example, it is contemplated that process10 may be performed by the layer wise masked exposure of the compositepowder to light, so that the portions of the powder to be fused areexposed to the light and the unfused portions are masked therefrom.

[0027] Upon completion of process 10, process 12 is then performed toremove the unfused or unsintered powder from around the article in theconventional manner. Such removal is commonly referred to as “roughbreak-out”, and generally involves the mechanical removal of the unfusedpowder to yield the green article. Further surface finishing of thegreen article may be performed at this time, if desired.

[0028] Upon completion of process 12, process 14 is then performed. Inprocess 14 the green article is placed in an oven or furnace, usuallypacked in inert powder packing made up of alumina or silica powders toprovide support during the subsequent heating steps. A lower meltinginfiltrant material is placed in the oven or furnace in contact with thegreen article. During process 14 the temperature of the oven or furnaceis slowly raised to a first temperature high enough to begin todecompose the polymer binders present. At these temperatures the metalhydrides present also begin to break down and release hydrogen gas inthe immediate environment of the decomposing polymers, the resultingreducing atmosphere accelerating the breakdown of the polymer fragmentsinto smaller fragments. This simultaneous breakdown of polymers andrelease of hydrogen leads to a much more complete removal of residualcarbon from the article skeleton, thereby reducing the likelihood of alater problem in these types of systems, that is surface blistering ofthe final infiltrated article due to residual carbon material forced tothe surface during final infiltration.

[0029] After process 14, process 16 is performed; the temperature of theoven or furnace is raised to increase the temperature of the articlefurther to begin a preliminary sintering of the composite articles toform a rigid skeleton. This now stronger article is often referred to asthe brown part or brown article.

[0030] After process 16, continuing to raise the temperature of the ovenor furnace performs process 18 when the infiltrant placed in the oven orfurnace in contact with the article melts and infiltrates the brownarticle, resulting in a fully dense article.

[0031] A preferred example of a composite powder to be used in theselective laser sintering process that is useful in connection with thisembodiment of the invention has a substrate of a stainless steel powder,such as spherical particles of 420, −53 micron, specification 2290stainless steel powder; a polymer binder system made up of approximately1% by weight of Ceracer 126A wax, available commercially from ShamrockSpecialty Products Group of Shamrock Technologies, Inc. of Newark, N.J.;approximately 1% by weight of Atofina 3501 UD natural nylon, availablecommercially from Atofina Chemicals, Inc. of Philadelphia, Pa.; andapproximately 1.25% by weight of a G-P 5546 phenolic, availablecommercially from Georgia-Pacific of Atlanta, Ga. The polymer binder ispreferably blended with the metal powder substrate particles. Inaddition, the preferred composite powder includes approximately 1% byweight of Monico titanium hydride powder −44 micron, availablecommercially from Monico Alloys of Los Angeles, Calif.

[0032] It should be recognized that other waxes, polyamides, andphenolics could be combined into workable systems for the purposes ofthis invention. In addition, other thermoplastics could be substitutedfor the polyamide and other thermosets for the phenolic. Potential metalhydrides that can be employed in the present invention include titaniumhydride, nickel-metal-hydride, magnesium hydride, lithium aluminumhydride, calcium hydride, sodium hydride, and sodium borohydride andcombinations thereof.

[0033] While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

We claim:
 1. A method of fabricating an article, comprising the steps offorming a green article by the selective laser sintering of a compositepowder, wherein said composite powder comprises metal and/or ceramicparticles, polymer particles, and particles of a metal hydride.
 2. Themethod of claim 1 wherein said metal hydride particles are selected fromthe group consisting of titanium hydride, nickel-metal-hydride,magnesium hydride, lithium aluminum hydride, calcium hydride, sodiumhydride, and sodium borohydride and combinations thereof.
 3. The methodof claim 1 further comprising using a steel powder in said compositepowder.
 4. The method of claim 3, further comprising using titaniumhydride as the metal hydride.
 5. The method of claim 2 wherein saidpolymer particles are selected from a group consisting of thermoplasticand thermoset polymers.
 6. The method of claim 5 further comprisingusing polyamide polymers.
 7. The method of claim 5 further comprisingusing phenolic polymers.
 8. The method of claim 1, further comprisingafter said forming step, heating said green article in a first heatingstep to a first temperature to decompose said polymer binder and saidmetal hydride.
 9. The method of claim 8, further comprising after saidfirst heating step heating said article in a second heating step to asecond temperature, the second temperature being above said firsttemperature, to sinter said composite particles to one another, forminga brown article.
 10. The method of claim 9, further comprising duringsaid second heating step, infiltrating said brown article with a secondmaterial.
 11. The method of claim 10 further comprising using a copperalloy as said second material.
 12. A preform green article formed byselective laser sintering comprising a composite powder, the compositepowder being fused and comprising metal and/or ceramic particles,polymer particles and metal hydride particles.
 13. The green articleaccording to claim 12 wherein the metal hydride particles are selectedfrom the group consisting of titanium hydride, nickel-metal-hydride,magnesium hydride, lithium aluminum hydride, calcium hydride, sodiumhydride, and sodium borohydride and combinations thereof.
 14. The greenarticle according to claim 12 wherein said composite metal powderfurther comprises a steel powder.
 15. The green article according toclaim 14 wherein said composite powder further comprises titaniumhydride.
 16. The green article according to claim 13 wherein saidpolymer particles are selected from the group consisting ofthermoplastic and thermoset polymers.
 17. The green article according toclaim 16 wherein said thermoplastic polymers further comprisepolyamides.
 18. The green article according to claim 17 wherein saidpolymer particles further comprise phenolics.